Electric vehicles and hybrid cars have been popularized recently because they are driven by a motor entirely or in part in order to protect the environment. The motors of those cars are powered by batteries, which are vulnerable to a quick discharge or recharge with a large amount of current and thus tend to change or deteriorate their characteristics. Therefore the electric current to be supplied to the motor is regulated particularly at a quick acceleration, so that the car sometimes cannot be sufficiently accelerated.
A car employing capacitors quickly dischargeable besides a battery is devised for allowing quicker acceleration than the acceleration with the battery only. This structure allows powering the motor at the quick acceleration with the capacitors in addition to the battery, so that the car can be accelerated quicker than it is accelerated only with the battery. An electrical storage device formed of capacitors is described hereinafter.
A voltage great enough to drive a motor is approx. 750V, which needs 300 pieces of capacitors rated at 2.5V coupled together in series, and parallel couplings can be used sometimes together with the series coupling in order to obtain necessary capacitance.
Since the capacitors have dispersion, and voltages applied to the capacitors are dispersed, a charge to the capacitors without considering the dispersion will substantially shorten the life of capacitors, and in the worst case, it breaks some capacitors. Thus methods of monitoring the states of a large number of capacitors and detecting an abnormality have been devised.
FIG. 11 shows a block circuit diagram illustrating a conventional method of detecting an abnormality in an electrical storage device. In FIG. 11, when DC power supply 1 charges a plurality of capacitors 2 rated at 2.5V, current detector 3 and voltage detector 4 measure respectively a current running through capacitors 2 and voltages across capacitors 2. Based on the measuring results, controller 5 regulates the charge to capacitors 2 and determines whether or not capacitors 2 are abnormal. The prior art discussed above is disclosed, e.g. in Unexamined Japanese Patent Publication No. 2003-274566.
The method discussed above can definitely detect an abnormal capacitor; however, when the capacitors are used auxiliary to a battery for driving a motor of a hybrid car, approx. as high as 750V is applied across entire capacitors 2 at the completion of charge, so that an extremely high voltage (approx. 750V) is applied to the whole circuit shown in FIG. 11. On the other hand, controller 5 calculates an amount of electric charge and an amount of electrostatic capacity based on the figures measured by current detector 3 and voltage detector 4 in order to determine the abnormality. For this purpose, an AD converter and a microprocessor are used, and those devices generally use DC 5V as their power source voltage. The block diagram shown in FIG. 11 tells that the results measured by current detector 3 and voltage detector 4, to both of which the high voltage is directly applied, are fed firsthand into controller 5, so that no one can deny that a defect or a malfunction, such as switch 6 and switch 7 are turned on simultaneously, can happen. If the high voltage is applied to controller 5 via current detector 3 or voltage detector 4, controller 5 can be malfunctioned and broken due to the high voltage far exceeding its allowable voltage.
Here is another problem: when capacitors 2 are discharged, a DC-DC converter is used for converting the specification into the one complying with the motor used in the hybrid cars. Noises generated by the converter can enter into controller 5 via current detector 3 or voltage detector 4. In this case, controller 5 sometimes malfunctions and cannot read the current or the voltage correctly.
On top of that, the foregoing method of detecting abnormality can surely detect an abnormal capacitor; however, when the capacitors are used auxiliary to the battery for driving the motor of the hybrid car, as many as 300 pieces of capacitors are needed as described previously. Thus switch 6 is provided to each one of capacitors 2, and the respective switches 6 are controlled by controller 5 via switch controller 7, so that at least over 300 long wires are needed between switch controller 7 and controller 5. As a result, the wirings become complicated and the weight of the car becomes heavier, which possibly lowers the excellent gas mileage of the hybrid car.